To resolve packages by name and version, npm talks to a registry website that implements the CommonJS Package Registry specification for reading package info.
Additionally, npm's package registry implementation supports several write APIs as well, to allow for publishing packages and managing user account information.
The official public npm registry is at https://registry.npmjs.org/. It is powered by a CouchDB database, of which there is a public mirror at https://skimdb.npmjs.com/registry. The code for the couchapp is available at https://github.com/npm/npm-registry-couchapp.
The registry URL used is determined by the scope of the package (see npm help 7 npm-scope). If no scope is specified, the default registry is used, which is supplied by the registry config parameter. See npm help npm-config, npm help 5 npmrc, and npm help 7 npm-config for more on managing npm's configuration.
Yes!
The easiest way is to replicate the couch database, and use the same (or similar) design doc to implement the APIs.
If you set up continuous replication from the official CouchDB, and then set your internal CouchDB as the registry config, then you'll be able to read any published packages, in addition to your private ones, and by default will only publish internally.
If you then want to publish a package for the whole world to see, you can simply override the --registry option for that publish command.
Set "private": true in your package.json to prevent it from being published at all, or "publishConfig":{"registry":"http://my-internal-registry.local"} to force it to be published only to your internal registry.
See npm help 5 package.json for more info on what goes in the package.json file.
No. If you want things to be public, then publish them into the public registry using npm. What little security there is would be for nought otherwise.
No, but it's way easier. Basically, yes, you do, or you have to effectively implement the entire CouchDB API anyway.
Yes, head over to https://npmjs.com/